Makefile: Fix 4.14.93 resolution

Commit ff858d82 ("kbuild: consolidate Clang compiler flags")
conflicts with commit b9b96462 ("ANDROID: Kbuild, LLVMLinux: allow
overriding clang target triple"). Remove the extra definition of
CLANG_FLAGS and rename CLANG_TARGET to CLANG_FLAGS as a proper
resolution.

As it currently stands, CLANG_FLAGS will not have the proper target when
cross compiling for aarch64-linux-android.

Change-Id: I176146ad613bd5e6187f3421f29c2ffcc9ceebc3
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>